Power amplifiers are typically used to increase the power level of an electrical signal. The relationship between the input power and the output power of a power amplifier is generally referred to as the “transfer function” of the power amplifier, and the magnitude of the transfer function is referred to as “gain.” Radio frequency (RF) power amplifiers oftentimes implement a gain control architecture that can be used to vary amplifier gain, and thus RF signal output power, for power leveling purposes.
In a voltage controlled power amplifier, gain control is typically implemented by detecting the voltage level of the power amplifier's RF output signal and comparing that voltage level to a reference signal. In a current controlled power amplifier, gain control is typically implemented by detecting the current level of the RF output signal and comparing that current level to a reference signal. Voltage-based gain control typically provides accurate control over the power amplifier's power leveling characteristics, even when variations in voltage standing wave ratio (VSWR) and output voltage occur. In contrast, current-based gain control typically exhibits poor power leveling characteristics when VSWR and output voltage vary.
There are disadvantages to using voltage-based gain control, however. For example, if a voltage controlled power amplifier is amplifying a time division multiple access (TDMA) signal, the power amplifier typically will ramp up power to transmit in allocated time slots (i.e. on specific channels), and then ramp down power in time slots allocated to other systems. If saturation occurs during this process, transient adjacent channel power (ACP) is oftentimes generated by the power amplifier. Transient ACP is known to adversely interfere with communication signals on adjacent TDMA channels. This phenomenon is commonly referred to as “splattering.”